The doping concentration of a DNW (deep N well) and an NW (N well) of a 40V isolation NLDMOS on a BCD700V, being shared with other devices, is unalterable. A DNW of an isolation NLDMOS functions to separate the body region from the substrate, thereby raising the doping concentration of the drift region and lowering the off-state breakdown voltage (off-By) of the device. As shown on FIG. 1, an NLDMOS structure of the prior art raises the off-state breakdown voltage by increasing the distance between the N well and the P well. Actual tape-out data shows that the on-state breakdown voltage of such a device is relatively low, the reason of which is that the doping concentration in the drift region near the channel side is reduced after the distance between the N well and the P well is increased and thereby cause a drop in the on breakdown voltage (on-BV).